Under the Transmission Control Protocol/Internet Protocol (TCP/IP), when a client provides a symbolic name (a Uniform Resource Locator or URL) to request access to an application program or another type of resource, the host name portion of the URL needs to be resolved into an IP address of a server for that application program or resource. For example, the URL (e.g., http://www.foundrynet.com/index.htm) includes a host name portion www.foundrynet.com that needs to be resolved into an IP address. The client first provides the host name portion to a local name resolver, which then queries a local Domain Name System (DNS) server to obtain a corresponding IP address. If a corresponding IP address is not locally cached at the time of the query, or if the time-to-live (TTL) of a corresponding IP address cached locally has expired, the DNS server then acts as a resolver and dispatches a recursive query to another DNS server. This process is repeated until an authoritative DNS server for the domain (e.g., foundrynet.com, in this example) is reached. The authoritative DNS server returns one or more IP addresses, each corresponding to an address at which a server hosting the application (“host server”) under the host name can be reached. These IP addresses are propagated back via the local DNS server to the original resolver. The application at the client then uses one of the IP addresses to establish a TCP connection with the corresponding host server. Each DNS server caches the list of IP addresses received from the authoritative DNS server for responding to future queries regarding the same host name, until the TTL of the IP addresses expires.
To provide some load sharing among the host servers, global server load balancing (GSLB) switches are sometimes used as proxies for authoritative DNS servers, together with one or more site switches each associated with one or more host servers. Each site switch provides the GSLB switch with current site-specific information (“metrics”) regarding access conditions to the host servers associated with the site switches. The GSLB switch then processes the addresses returned by the DNS server using the metrics compiled from the site switches and provides an ordered address list having the optimum address for access listed at the top. An example of a GSLB system and description of associated metrics are disclosed in U.S. application Ser. No. 10/376,903, entitled “GLOBAL SERVER LOAD BALANCING,” filed Feb. 28, 2003, assigned to the same assignee as the present application, and which is incorporated herein by reference in its entirety.
There are other examples of server load balancing systems and techniques, apart from those that use GSLB. At least some of these GSLB and/or other load balancing systems use round trip time (RTT) measurements in determining an optimum address to provide to a client. For a GSLB system, the site switches gather RTT and report these measurements to the GSLB switch. The GSLB switch maintains a database for these RTT measurements. In particular, each entry in this database includes the following information:                Network prefix for which the RTT was reported;        The site switch that reported this RTT; and        The RTT value.        
Whenever a site switch reports a new RTT value for a network prefix, this value is smoothed into the existing RTT value in the RTT database. Generally, the most common smoothing algorithm used is to compute the effective RTT value as a sum of 90% of the existing RTT value and 10% of the new RTT value, for example. This algorithm is very simplistic in nature and not able to adapt quickly to large variances in RTT values or changes in RTT measurements. Further, it does not provide flexibility to customize the RTT smoothing based on the RTT characteristics of different networks.